1. Technical Field
This invention relates to intake and exhaust valves for an internal combustion (IC) engine, and more particularly, to such valves formed of a fiber reinforced ceramic matrix composite (FRCMC) material and methods for making them.
2. Background Art
In a typical four-stroke gasoline-powered internal combustion (IC) engine, an intake valve is designed to open during the intake stroke of the engine to allow a fuel mixture to enter the engine cylinder. Once the intake stroke is complete, the intake valve closes to seal the engine cylinders so that the fuel mixture can be compressed during a compression stroke of the engine. Upon completion of the compression stroke, the fuel mixture is ignited causing an expansion of gases which push the piston down in the cylinder. At this point an exhaust valves opens to allow exhaust gases to be expelled from the cylinder as the piston moves up in the cylinder (i.e. the exhaust stroke). Other types of IC engines work similarly, some having only one valve per engine which acts as both the intake and exhaust valve, while others have multiple intake and exhaust valves per cylinder. The basic structure of a valve, as shown in FIG. 1, includes a stem 12 and a head 14. The head 14 has a bottom surface 16 which is directed toward the interior of the cylinder of an IC engine when the valve is installed therein. In addition, the head 14 of the valve has tapered side surface 18 which interfaces with the valve seats of the engine in order to form the aforementioned seal when the valve is closed.
The valves in any type of IC engine operate in a violent environment characterized by extreme operating temperatures and temperature variations, excessive gas pressures, corrosive fuel components, and intense hammering caused by the opening and closing of the valves. As such valves need to be very tough and durable. Most valves are made of metal, often involving complex multi-part constructions and exotic alloys. For example the head of the valve could be made of a high temperature resistant alloy to withstand the temperatures found inside the cylinder, whereas the stem could be made of a alloy which is stiffer and provides good bearing qualities. These characteristics are desirable as the stem resides within a valve guide which assists in aligning the valve and sealing the lower portion of the valve and engine cylinder from oil in the upper part of the engine. Thus, the stem must withstand the reciprocal motion between it and the valve guide. Valves also often have coatings or hollow portions filled with heat dissipating salts for the same reasons.
Although metal valves can be made more durable by the use of various alloys, coatings, etc., they still have limits as to the temperatures which they can withstand. However, the performance and fuel economy of an IC engine can be improved by increasing the temperature of the combustion chamber (i.e. the cylinder) beyond the limits of metal valves. This improvement in performance and fuel economy results because the higher chamber temperatures cause a more complete burning of the fuel. Therefore, more energy is released and less fuel is required to drive the engine. However, because of the temperature limitations of metal valves, operating at these higher temperatures has not been possible because, among other things, the valves would fail (which is often termed xe2x80x9cburning the valvesxe2x80x9d). Typically, the valves fail because the increased temperatures cause the valves to expand to a degree that they no longer form a seal with the associated valve seat. This mismatch occurs because the head of an IC engine, which includes the valve seats, is cooled, typically by water or cooling fluid circulating through channels in the head. As the head is cooled, the valve seats do not tend to expand significantly while the engine is operating. However, the intake and exhaust valves of the engine are not cooled like the valve seats. As a result, the valves expand as the engine heats up. The tapered surface of the head of a valve which interfaces with the valves seats is often specially ground by an expensive and complex process referred to as a triple grind. The special grinding ensures the valve head will seal with the valve seat even though it expands as the engine heats up. However, if the temperatures become too great, even special grinding cannot accommodate the expansion of the valve head. If the valve head expands to the point that it not longer seats into the valve seat, the valve and the valve seat can become physically damaged by the mismatch, and hot exhaust gases can leak through any gaps formed between the two structures causing localized burning of the metal forming the valves and valve seats. The resulting damage can increase pollutants emitted by the engine, reduce engine performance, or even cause the engine to fail completely.
One attempt to resolve the problems associated with conventional metal valves has been to make them from a monolithic ceramic material. For example, valves formed of silicon nitride are commercially available. Monolithic ceramic valves have the advantage of being extremely resistant to damage by heat in that ceramic material has a low thermal conductivity and will not readily absorb heat. In addition, ceramic materials are thermally stable in that they exhibit a low coefficient of thermal expansion and so do not expand significantly as the temperature increases. Thus, even at higher engine operating temperatures, ceramic valves will not expand to an extent which would jeopardize their sealing with the valve seats. As a result there is no damage or burning of these structures. Additionally, monolithic ceramic valves made of materials such as silicon nitride tend to be hard, while at the same time having external surfaces which exhibit a low coefficient of friction (i.e. slipperiness). The hardness of the ceramic material is advantageous as it makes the bottom face of the head portion of the valve resistant to the violent environment of the cylinder of the engine. The low coefficient of friction or slipperiness of the material is advantageous for two reasons. First, the slipperiness of the surface facilitates the sliding of the tapered surface of the valve head in and out of the valve seat without causing any abrasive grinding between the interfacing surfaces or a valve sticking condition. In addition, the stem of the valve which resides within the aforementioned valve guide would advantageously have a slippery surface. For example, the slipperiness of a valve made from a silicon nitride ceramic material results in the valve stem sliding easily within the valve guide. As such the stem and the valve guide will not be damaged which could otherwise cause a misalignment of the valve and/or oil to drip down the valve stem from the upper part of the engine.
Ceramic valves also have another advantage in that they weigh less than metal valves. The weight and size of the entire valve train is effected by the weight of the valves. Lighter valves allow the use of a smaller, lighter valve springs, which in turn means the camshaft does not have to be as stiff. Thus, a smaller, lighter camshaft can be employed. In addition, the rocker arms and push rods (if employed) can be smaller and lighter as they do not have as much weight to push around. Thus, the entire valve train can be made lighter by employing lighter valves. Reducing the weight of the valve train makes for a lighter, more efficient engine.
However, monolithic ceramic valves present unique problems of their own. Monolithic ceramic structures tend to be porous and brittle, and extremely difficult to form without structural flaws. These structural flaws make the material subject to cracking. Thus, the monolithic ceramic valve is susceptible to catastrophic failure when impacted, or otherwise subjected to even moderate forces. They are also strain intolerant and cannot be deflected more than 0.1 percent without being fractured. These are very undesirable characteristics for a moving part such as a intake/exhaust valve. Further, if the ceramic valve fails, broken pieces of the valve can cause further damage to a working engine.
Monolithic structures are also difficult to manufacture. For example, production yields for ceramic valves is often 50 percent or less. This, of course, increases the cost of monolithic ceramic valves.
Accordingly, there is a need for IC engine valves which exhibit the high temperature resistance, light weight, minimal thermal expansion, hardness and slipperiness of a monolithic ceramic valve (such as one made of silicon nitride), but which are fracture resistant and easy to manufacture with high yield rates.
Wherefore, it is an object of the present invention to provide a valve for an IC engine which is flaw insensitive, strong and ductile, so as to be fracture resistant and capable of withstanding the strains encountered in an operating engine without failing.
Wherefore, it is another object of the present invention to provide a valve for an IC engine which exhibits a low thermal conductivity so as to be capable of withstanding high temperatures, a low coefficient of thermal expansion, and is light weight in comparison to metal valves.
Wherefore, it is yet another object of the invention to provide a valve for an IC engine which exhibits a high degree of hardness and a surface having a low coefficient of friction.
Wherefore, it is still another object of the invention to provide a valve for an IC engine which is easy to manufacture and capable of approaching a yield rate of 100 percent.
The above-described objectives are realized with embodiments of the present invention directed to a fracture-resistant, thermally stable intake or exhaust valve for an internal combustion (IC) engine. The valve has a stem portion and a head portion, both of which are formed of fiber reinforced ceramic matrix composite (FRCMC) material. This FRCMC material generally includes a polymer-derived ceramic resin in its ceramic state, fibers, and filler materials.
The pre-ceramic resin used to form the FRCMC material can be any commercially available polymer-derived ceramic precursor resin, such as silicon-carboxyl resin or alumina silicate resin, and the fibers are preferably at least one of alumina, Nextel 312, Nextel 440, Nextel 510, Nextel 550, silicon nitride, silicon carbide, HPZ, graphite, carbon, and peat. The fibers are also preferably coated with an interface material which increases the ductility exhibited by the FRCMC material. Specifically, the interface material preferably includes at least one 0.1-0.5 micron thick layer of at least one of carbon, silicon nitride, silicon carbide, silicon carboxide, or boron nitride.
The fibers forming the stem portion of the valve preferably take the form of a continuous, woven fiber rope. The fiber rope is continuous because each fiber therein runs the entire length of the rope. In addition, the percent by volume of the stem portion constituting the fiber rope is preferably as large as possible. The large percentage of fiber making up the stem, in combination with the continuous, woven structure of the fiber rope, gives the stem portion a high degree of ductility and stiffness so as to survive the hammering causes by the opening and closing of the valve when the engine is running, and to maintain the alignment of the valve head. In addition, it is preferred that the stem portion have an outer layer which includes filler material which lowers the coefficient of friction exhibited by the surface. This makes the stem portion slippery and facilitates its movement through a valve guide. Preferably, the filler material is at least one of carbon or silicon nitride. The degree to which the coefficient of friction is lowered is dependent upon the percentage by volume of filler material making up the outer layer of the stem, and as such it is preferred that this percentage be large enough to produce the desired slipperiness.
The head of the valve must withstand the violent environment associated with the inside of the cylinder of the engine, as well as smoothly interfacing with a valve seat to seal the cylinder. This interfacing must be accomplished without damage to the valve seat or sticking of the valve within the seat. In view of these requirements, it is preferred that the head portion include filler material which increases its hardness and decreases its coefficient of friction. Preferably, the filler material is at least one of silicon nitride, boron carbide, boron nitride, or silicon carbide. This filler material would preferably constitute a sufficient percentage of the valve head to ensure the head is hard enough to survive the cylinder environment, as well as ensuring the head""s surface is slippery enough to ensure a smooth interface with the valve seat.
It is noted that in the preferred embodiment of the valve, the woven fiber rope associated with the stem portion has a end which is frayed (i.e. un-braided) and which extends into the upper region of the head portion. This strengthens the connection between the stem and head portions of the valve, above that which can be provided by the ceramic matrix of the FRCMC material alone.
Forming the IC engine valve from the aforementioned FRCMC material has significant advantages. For example, FRCMC material is highly temperature resistant so that the operating temperatures of the IC engine can be increased without destroying the valve. In addition, FRCMC material is temperature stable in that it does not expand significantly with increasing temperature. Thus, the head of the valve will not expand to a degree that causes damage to it or the valve seat. FRCMC material is also ductile, thus making the valve fracture resistant and capable of withstanding the hammering caused by the opening and closing of the valve, as well as the thermally-induced strains caused by wide temperature variations associated with an IC engine. The FRCMC material is also flaw-insensitive in that any flaw within the structure of the valve will not result in cracking and failure. In addition, FRCMC valves are considerably lighter than the existing metal valves. This provides an opportunity to reduce the weight of the overall valve train, thereby increasing engine performance. As mentioned above, the addition of certain filler materials allows the FRCMC material to be tailored to exhibit a desired degree of harness and/or a desired coefficient of friction. This tailoring can also be localized such that the degree of hardness and the coefficient of friction can vary from one part of the valve to another. Finally, it is noted that FRCMC material, being a mixture of pre-ceramic resin, fibers and filler materials in its raw state, is readily formable via a variety of methods. This makes the engine valve according to the present invention easy to manufacture. The ease in manufacturing especially applies to the preferred method of compression molding the IC engine valves.
In addition to the just described benefits, other objectives and advantages of the present invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.